FIG. 1 shows, diagrammatically, a typical arrangement for the control of fluid extraction from each of, in the example, four wells of a hydrocarbon extraction field. Such arrangements are typical for a field of subsea wells. The field is connected to an umbilical 1 terminated by a seabed umbilical termination assembly (UTA) 2 which, typically, supplies control signals to subsea control modules (SCM's) 3 mounted on Christmas trees (XT's) fitted to the wellheads. Sometimes, the UTA 2 feeds control signals directly to an SCM 5 mounted on a manifold 6 which controls the fluid extraction output from the field. Alternatively, the manifold 6 can be controlled by an SCM 3 mounted on one of the Christmas trees or its functions shared between several SCM's on more than one tree. Typically, the umbilical 1 also feeds hydraulic fluid under pressure to operate hydraulically operated devices such as chokes and valves, plus electric power supplies to the SCM's, and sometimes electric power to operate electrically operated devices as well. The umbilical 1 also carries electrical signals from sensors fitted to the system, such as pressure and temperature sensors, to provide monitoring data to assist the operator in controlling the field. The other end of the umbilical 1 terminates on a surface vessel or a platform or sometimes on land, which carries the controlling equipment and interfaces to the operator. The extracted fluid output from each well is fed to the manifold 6 and then to the field output flowlines 7 to the surface vessel, platform or land base.
One disadvantage of this system is that the Christmas trees 4 and the manifold 6 are heavy and complicated by the attachment of a SCM to each of them, thus making them expensive to manufacture and install. A further disadvantage is that the UTA 2 is also heavy and complicated.